Yes, most major airline loyalty programs in 2026 allow for "retroactive mileage claims," but there is a strict time limit. For the vast majority of carriers like United, Delta, American, and ANA, you have a six-month (180-day) window from the date of the flight to request missing credit. To do this, you typically need your ticket number (a 13-digit code) and your boarding pass or "Record Locator" (PNR). Some airlines allow you to claim miles for flights taken before you even joined the program, but this "look-back" period is usually much shorter, often only 30 days. In 2026, many apps have a "Scan Boarding Pass" feature that automates this process. It is important to note that you cannot claim miles if the flight was already credited to a partner airline's program. If you are flying on a partner (e.g., flying Lufthansa but wanting United miles), the process can take up to 15 days to reflect, so airlines usually ask you to wait at least 7 days after the flight before submitting a manual claim.